With the new edition of Age of Sigmar out there, there's going to be new army books for the various factions. In this case, since they're the starter force armies, the first two Battletomes are for the Nighthaunt and Stormcast Eternals. There's also several new units, a set of dice, and templates to use for your various magic effects out on the battlefield.

From the website:

An indomitable new malevolence has stirred an ancient terror. No longer content to hunt civilisation’s edges, phantasmal forces gather into spectral armies. This is no mere haunting, but a shock assault that seeks nothing less than to destroy life itself. From the darkest pits of Shyish, the Realm of Death, comes a storm of ethereal figures. These wraiths fight to send fresh souls screaming down into the darkened realms from whence the dead emerged, taking cruel pleasure in knowing the torments that await their victims below…

The Stormcast Eternals – supernatural warriors forged in Azyr, each the embodiment of Sigmar’s storm. The heavens roar and the sky crashes, split by searing bolts from above. With an explosion of lightning and a booming thunderclap, the forces of the Stormcast Eternals manifest in reality and plunge into battle. Energised by the magical tempest, they launch their attack, weapons wreathed in celestial energy. Razor-edged blades rise and fall, and heavy hammers swing in an onslaught that batters down all before it. Theirs is the fury of the heavens unleashed.